The Homelite 25 or 30cc engines are good, with the 30cc having a decided edge but they are a little harder to find. If there's an "OS" of conversion engines, I would say it is the Echo/Kioritz engines. Light weight, good power, and usually easy to convert. They aren't as common as the other engines as they are in upscale, more professional type equipment, and as such are also sometimes a little difficult to find. Another thought is the Ryobi 31cc engine. It is popular, easy to find, and easy to convert. It also has the benefit of being narrow because the carb and muffler are mounted on the back of the engine. This works nicely in airplanes with narrow cowls, like Cubs, Spacewalkers, Tiger Moths, etc.

If you think you might like a nice large scale biplane, check out the big Tiger Moth at www.nitroplanes.com. I'm doing one right now with a Ryobi. The Ryobi fits entirely within the cowl, and I think it will fly this plane nicely. It has a 78" wingspan, and is nicely built.

Good luck and have fun,
AV8TOR